The Alien and Sedition Acts 1798--four acts created by Congress in 1798 during the height of the Federalist party power, with John Adams as President following after George Washington--were important for (1) what they responded to, for (2) what they were, for (3) what they violated, and for (4) what they led to. The Adams presidency never did make use of the Alien acts, but the Sedition Act was infamously used during the Adams presidency to quell protests against government policy and against the Congress and President.
What they responded to: The XYZ affair (1797), aimed at Adams' Federalist government, involved three unnamed French representatives--Messrs. X, Y, and Z representing Charles-Maurice de Talleyrand-Périgord--who, in the wake of the contested Jay’s Treaty (1795), demanded an apology from Adams and two payments of large sums of money (1.2 million livres and 32 million florins) in order for peace negotiations between the American representatives (Charles Cotesworth Pinckney, Washington's representative and a Federalist, Elbridge Gerry, a Massachusetts Republican, and John Marshall, a Virginia Federalist) and the French government to proceed toward reparation of French-American relations. When the United States learned of this demand, which constituted nothing less that the solicitation of bribe money, the government and the citizenry were outraged and Congress began taking legislative action to secure American safety and to prepare for military retaliation, especially since the French, following the example of the British, had already been seizing American vessels on the high seas (316 U.S. vessels in 1796).
What they were: The four Congressional Acts--which now are seen as a clear violation of Constitutional powers--comprised three acts aimed at immigration and one act aimed at disloyalty to the Unites States and the U.S. government, or "sedition" (sedition: behavior, speech or writing that incites rebellion against the ruling power of government).
Naturalization Act: extend citizenship requirement to 14 years from 5 years
Alien Act: Presidential power to imprison of deport friendly foreigners
Alien Enemies Act: Presidential power to imprison or deport enemy foreigners
Sedition Act: silencing of Republican presses and dissension through forbidding disloyal words or actions
Historians now agree that these acts were aimed at destroying the power of Thomas Jefferson's Republican Party, thus at securing a one party system in the United States, and at curtailing the freedom of speech for dissenting parties based upon the idea that dissent and loyalty are opposing positions: if you oppose government policy, you are disloyal to the government, therefore guilty of sedition and inciting rebellion against the government. One other factor behind the acts was the number of French emigres, who aligned themselves with the Republican Party, especially those emigres from the French West Indies who had fled the effects of the French Revolution and its subsequent "Terror."
What they violated: Whether disallowing citizenship with the aim of eliminating a two-party and the right to free expression of ideas and beliefs by immigrants and citizens or whether denying freedom of expression in the press, historians now agree that the Acts violated Constitutional First Amendment rights of freedom of speech and of the press.
What they led to: Ultimately, what the Acts led to was the reverse of what they hoped to put an end to. The Acts led to the Kentucky and Virginia Resolutions in which Jefferson and Madison set forth strong cases against Federal control of legal policy and a strong case for State rebuke and repudiation of Federal law, a precedent later used by the South in seceding from the Union. The Acts further led to the election of a Republican government in the 1800 election when Federalists and John Adams were voted out of office and Republicans and Thomas Jefferson were voted into office.
Election of 1800
Alien and Sedition Acts Documents
The Alien and Sedition Acts
Library of Congress, Alien and Sedition Acts Documents